A 'very good' high % kick first is advantageous because it gives you time to get into the net as the ball is in the air longer and as I said a 'very good' kick serve is not easy to return, so it should give you easier volleys.

The thing with a flat first serve is while most of the time if it is well placed it might be hard to return, if someone is able to block it back it will come back very fast at you not giving you as much time to get to net.